MILES AND GRINNELL GLACIERS 429^
years up to 1910, a remarkable maintenance of a stagnant ice mass due to the protective influence of ablation moraine and vegetation.
Advance Before 1888. The ages of trees on the southern terminal moraine and in the zone of thick ablation moraine on the northern lobe make it clear that at some time over 22 years before 1910, there was a period of advance, breaking up the ice on the northern bulb to within three quarters of a mile of Abercrombie Rapids and building the southern terminal moraine. That this took place after 1884 and 1885 is suggested by Aber-•crombie's photograph and Allen's map which, though generalized, shows no southward extension of the bulb. This expansion of the southern margin to within 400 feet of the site of the railway bridge clearly came after 1885 and before 1888, and may have begun in the former year, for Allen's map shows the glacier nearly out to the bridge site. If this advance occurred in 1885 or 1886, it was exceptionally rapid, since in 1884, Aber-•crombie was able to travel up the southern margin of the glacier, and in April, 1885, Allen noted that the drift graded indistinguishably into the ice of the southern edge of the .glacier.
Recession Up to 1908. A general retreat began by at least 1888 and had progressed .so far that the lake was about a mile wide at the time of Hayes' visit in 1891.
The 5 or 6 mile retreat mentioned by Abercrombie on his second visit to Miles Glacier in 1898 is probably an exaggeration, for his map does not indicate that the lake was over a mile wide. This map does not show the details as well as Hayes' map and may represent the condition in 1884 rather than that in 1898. There was doubtless marked retreat, however, by 1898.
By 1900, when Schrader and Spencer each photographed the ice front on different dates in October, and Witherspoon made his map, the retreat of the ice cliff had been considerable. A careful comparison of the 1900 photographs with the conditions in 1910 indicated that the ice front had then receded so far that the lake had assumed nearly its present width, that is, different parts of the glacier front had retreated from l£ to 2 miles from the western terminal moraine of the southern part of the bulb.
From 1900 to 1907, when Moffit visited Miles Glacier, retreat went on rapidly, different portions of the ice cliff melting back 1700 to 8900 feet during the 7 years interval. This recession was carefully determined in 1910 in two ways. First, we occupied the photographic stations of Schrader and of Moffit with their photographs in hand and made instrumental observations of the amount of recession, by projecting the glacier termini -against the mountain wall behind. This retreat is shown graphically on the photograph (PI. CLXVIEI), being marked on the lobe which retreated 1700 feet. The second method of determination was by comparing maps made in 1900 by the Geological Survey and in 1906 by the railway engineers, based on surveys in 1905 and 1906. These maps show that while the lobe mentioned above was retreating 1700 feet during the 6 years from 1900 to 1906, another lobe, farther south, receded nearly 8900 feet. The larger part of the retreat was between 1905 and 1906.
That there was additional recession of 1620 feet between 1906 and 1908 is shown by another map of the railway engineers. The position of the ice cliff at this stage is shown in a photograph by E. A. Hegg, from the western terminal moraine.
Advance in 1909 and 1910. The change from retreat to advance came in 1909 and 1910 "when there was a forward movement of over 4000 feet along one of the lobes. This was determined with some precision, for the railway engineers mapped the whole lake front